Which detection method finds exoplanets by watching a star dim as a planet crosses in front?

Transit photometry finds exoplanets by watching a star dim as a planet crosses in front of it.

During a transit, the planet blocks a small fraction of the star’s light, producing a repeating dip in a brightness-versus-time graph called a light curve. The depth of that dip reveals the planet’s size relative to its star, while the interval between dips gives the orbital period.

The method is powerful but depends on alignment: the planet’s orbit must carry it across the star’s visible disk from Earth’s viewpoint. Most planetary systems therefore do not show transits to us. Transit observations first detected HD 209458 b’s crossings in 1999, while OGLE-TR-56b became the first planet discovered using the transit method in 2002.

A common mix-up is confusing transit photometry with radial velocity. Transit data primarily measure radius; radial-velocity data measure the star’s motion and provide a mass estimate. Together, the two methods can reveal a planet’s density.